The present invention relates to a power semiconductor device in which the control electrode section has a solderless contact construction and, more particularly, to an improvement of the solderless contact construction.
In recent years, remarkable progress has been made in increasing the power capacity of a power semiconductor device. In the case of a gate turn-off thyristor (hereinafter referred to as a GTO thyristor), the maximum turn-off current I.sub.TGQ was 800A in the early 1980s. Thereafter, the maximum turn-off current I.sub.TGQ was increased every year, and by the middle of 1980s, a GTO thyristor, the maximum turn-off current of which was increased up to 2,700A, because commercially available. With this increase in maximum turn-off current I.sub.TGQ, the maximum value I.sub.RGP of the reverse gate current for turning off a GTO thyristor was also increased every year; it increased from 200A to as large as 700A.
With an incrase in the power capacity of power semiconductor devices such as a power transistor, an ordinary thyristor, a GTO thyristor etc., a solderless contact construction has come to be widely employed for connecting the element-controlling electrode (e.g., the gate electrode of the thyristor, and the base electrode of the transistor) and the external lead to each other. Solderless contact construction is usually adopted in the fabrication of a power semiconductor device with increased power capacity, such as a GTO thyristor the turn-off current of which exceeds 1000A.
FIG. 1 is a sectional view illustrating the solderless contact construction of the gate electrode section of a conventional GTO thyristor.
In FIG. 1, reference numeral 1 represents a semiconductor pellet on which a GTO element is formed. Gate electrode 1a formed of Al (aluminium) is located in the center of pellet 1, and cathode element 1b surrounds gate electrode 1a. Reference numeral 2 represents a pressing cathode formed of Cu (copper). Pressing cathode 2 urges cathode element 1b, with buffer plate 2a and foil 2b, both formed of Mo (molybdenum), interposed therebetween. Reference numeral 3 represents a pressing gate which is insulated from pressing cathode 2 by Telfon ring 3a and ceramic disk 3b. Generally, pressing gate 3 is formed of a base material of Cu plated with Ni (nickel) and Ag (silver). The Ag layer generally has a thickness of several .mu.m.
As is shown in FIG. 1, pressing gate 3 is urged by coil spring 4 placed on spring seat 2c of pressing cathode 2, to urge gate electrode 1a of the GTO thyristor via pressing gate 3.
In a GTO thyristor which uses the solderless contact structure shown in FIG. 1 and the maximum turn-off current of which is 2,000A, the pressing gate with a pressing area of 52 mm.sup.2 is urged, by the coil spring with a pressure of 1.5 kg, to urge the gate electrode, formed on the pellet. Accordingly, the pressure per unit area is about 0.03 kg/mm.sup.2.
It has recently been found, however, that the conventional solderless contact construction is inappropriate to a GTO thyristor with an increased power capacity. If the GTO thyristor, the maximum value I.sub.RGP of the reverse gate current of which exceeds 300A, is used continuously over a long period of time, the contact resistance between the gate and the cathode increases with time. Within about 3-5 years, the gatecathode resistance increases considerably. In the end, the Al layer, which constitutes gate electrode 1a on the pellet, is melted by the heat generated due to the increased resistance of the pressed sections. As a result, the breakdown voltage between the anode and the cathode of the GTO thyristor is adversely affected. This pnenomenon can be observed with respect to not only the GTO thyristor but a power transistor with a pressed base.